(1) Field of the Invention
This invention relates to a method of adjusting a pressure sensor, and more particularly to a method of adjusting a pressure sensor for use in detecting the pressure of a medium to be measured, such as a refrigerant gas used in an automotive air conditioner.
(2) Description of the Related Art
An automotive air conditioning system imposes a relatively large load on an engine that drives the system, and hence it is considered that the engine should be controlled optimally in accordance with an operating state of the air conditioning system. In the automotive air conditioning system, information necessary for engine control includes respective pressures on the suction side and the delivery side of a compressor directly driven by the engine, and the pressures are detected by using pressure sensors.
In general, it is required that the pressure of a refrigerant gas is electronically detected, so as to reduce the size and weight of the pressure sensors for the automotive air conditioner. To this end, there is employed a pressure sensor produced by forming a thin diaphragm portion at a central portion of a silicon substrate, and printing a conductor pattern and a resistance pattern for wiring on the surface of the diaphragm portion to form semiconductor strain gauge resistances, thereby forming a Wheatstone bridge using the resistances.
The pressure sensor of the above-mentioned kind is suitable for mass production by the IC manufacturing process, but changes in temperature considerably affect the characteristics of the strain gauge resistances. As a solution to this problem, i.e. with a view to compensating for the temperature characteristics, Japanese Patent Publication (Kokoku) No. 59-41134 proposed a pressure sensor in which a resistance circuit for correction of a temperature-dependent error is formed on a non-deformable portion of the diaphragm portion, which is insensitive to pressure, and a pressure signal by gauge resistances is corrected by the resistance value of the resistance circuit, and then output.
Another pressure sensor is disclosed in Japanese Laid-Open Utility Model Publication (Kokai) No. 2-89339, which integrally includes a diaphragm portion having strain gauge resistances formed thereon, and an amplification circuit connected to the output terminal of the gauge resistances on the diaphragm portion, for amplifying an output voltage varying with a change in the resistance value of the gauge resistances to output the resulting voltage.
In the pressure sensor constructed as above, characteristics of an offset, temperature drift, and a span voltage of the gauge resistances of the diaphragm portion are adjusted, and further, characteristics of an offset and the like of the amplification circuit are adjusted, to thereby reduce a measurement error as a whole.
However, in the conventional pressure sensor, it is necessary to carry out separate adjustment operations on the diaphragm side and on the amplification circuit side, in a manner such that the diaphragm portion has the predetermined characteristics, and the amplification circuit also has its characteristics adjusted, which complicates the process of adjusting the pressure sensor.
Further, to reduce the measurement error, there are formed adjustment resistance circuits for compensating for the resistance values of the gauge resistances, and a linear dividing line is formed on each adjustment resistance circuit by laser trimming, whereby a total output voltage from the circuit is adjusted.
However, in a small-sized sensor which cannot allocate a large area for the adjustment resistance circuits, the length of a dividing line by the laser-trimming does not necessarily correspond to the resistance value. Particularly when a resistance formed on a wiring pattern suffers from print deviation, it is difficult to adjust a value of voltage to be applied to the gauge resistances, because adjustment of the output voltage by laser trimming is limited.